Re: 2.6.24-rc4-git5: Reported regressions from 2.6.23

Ingo Molnar
2007-12-11T08:43:58Z

* Arjan van de Ven <arjan@infradead.org> wrote:

> > That sounds like a big problem.
>
> it'll get way worse going forward. (but even on todays systems, the
> tsc no longer represents frequency, but is some fixed clock totally
> unrelated to cpu frequency)

X86_FEATURE_CONSTANT_TSC CPUs (all modern Intel CPUs) should be fine -
we dont do any TSC frequency fixups for them. The loops_per_jiffy fixup
looks like this:

if (!(freq->flags & CPUFREQ_CONST_LOOPS))
cpu_data(freq->cpu).loops_per_jiffy =
cpufreq_scale(loops_per_jiffy_ref,
ref_freq, freq->new);

i.e. X86_FEATURE_CONSTANT_TSC excluded. The sched_clock() scaling factor
is modified like this:

if (!(freq->flags & CPUFREQ_CONST_LOOPS)) {
tsc_khz = cpu_khz;
preempt_disable();
set_cyc2ns_scale(cpu_khz, smp_processor_id());

so here X86_FEATURE_CONSTANT_TSC is excluded again. So the whole
frequency scaling issue will become a pure legacy issue only with time.

Ingo